A backhaul (hereinafter, referred to as a mobile backhaul) between radio base stations and a mobile core network may include a plurality of links. FIG. 10 is a block diagram illustrating one example of the mobile backhaul. The mobile backhaul is divided into an access link, an aggregation link, and a metro link. The access link is a link that accommodates a radio base station connected to a relay apparatus disposed in an access area. The aggregation link is a link that accommodates a plurality of radio base stations connected to relay apparatuses disposed in the access area and a radio base station (not illustrated in FIG. 10) disposed in an aggregation area. The metro link is a link in a metro area and is a link to transfer a large amount of mobile data traffic of the plurality of radio base stations transferred via the aggregation link to a mobile core network. Hereinafter, a communication resource in the backhaul will be referred to as a backhaul resource. Further, a traffic load in the backhaul resource will be referred to as a backhaul resource load.
As a method for distributing backhaul resource loads, there is, for example, MLB (Mobility Load Balancing) executed between eNBs (evolved Node Bs). The eNB is a radio base station corresponding to LTE (Long Term Evolution). MLB is executed in accordance with procedures indicated in 1 to 4 described below.
1. Via an X2 link between eNBs, there is executed resource status reporting processing, specifically, Resource Status Reporting Initiation Procedure described in technical specifications (TS 36.423 Version 11.5.0) of 3GPP (3rd Generation Partnership Project).
FIG. 11 is a sequence diagram illustrating resource status reporting processing between eNBs. In the resource status reporting processing, as illustrated in FIG. 11, a resource status request message (X2: RESOURCE STATUS REQUEST message) is transmitted from an eNB 1 to an eNB adjacent thereto (hereinafter, referred to as an adjacent eNB), i.e. an eNB 2. Then, from the eNB 2 to the eNB 1, a response message (X2: RESOURCE STATUS RESPONSE message) to the resource status request message is transmitted.
FIG. 12 is an illustrative diagram illustrating a structure of the RESOURCE STATUS REQUEST message. The eNB 1 designates load information of the adjacent eNB (eNB 2) that establishes an X2 link as a measurement object in the RESOURCE STATUS REQUEST message. Specifically, as illustrated in FIG. 12, the eNB 1 designates “TNL (Transport Network Layer) load Ind Periodic” as the parameter “Report Characteristics (measurement object items)” of the RESOURCE STATUS REQUEST message. Thereby, the eNB 1 becomes able to periodically receive load information of the eNB 2. It should be noted that in “Report Characteristics,” up to four pieces of load information can be designated. In the present example, as one thereof, “TNL load Ind Periodic” is included.
2. The eNB 1 receives a resource status update message (X2: RESOURCE STATUS UPDATE message) from the adjacent eNB.
FIG. 13 is an illustrative diagram illustrating a structure of the RESOURCE STATUS UPDATE message. The RESOURCE STATUS UPDATE message received by the eNB 1 includes “S1 TNL Load Indicator” as illustrated in FIG. 13. “S1 TNL Load Indicator” is load information of an S1 transport network layer (hereinafter, referred to as S1 network load information). The S1 network load information is information indicating a backhaul resource load of a link, i.e. an access link between an eNB and a first-hop relay apparatus from the eNB. A value of the S1 network load information is indicated by any one of four stages of “LowLoad,” “MediumLoad,” HighLoad,” and “Overload.”
3. The eNB 1 changes, when a load of TNL exceeds a certain threshold, a handover attribute value (a value of a parameter for controlling Handover described in the technical specifications) to switch a cell where a radio terminal is located from a cell of the eNB 1 to a cell of an adjacent eNB having a low load, i.e. to execute Handover. At that time, the eNB 1 designates the cell of the adjacent eNB having a low load as a handover destination on the basis of the S1 network load information received from the adjacent eNB. Herein, it is assumed that the eNB 1 designates a cell of the eNB 2.
4. The eNB 1 handovers the cell where the radio terminal is located from the cell of the eNB 1 to the cell of the eNB 2 to distribute backhaul resource loads.
PTL 1 describes a technique in which in a radio communication system where an eNB communicates using a plurality of carriers, the eNB exchanges carrier configuration information, load information, and a resource usage status with another eNB to avoid inter-cell interference and execute load distribution among carriers.